insidious
adj.adj. describing something that spreads slowly and quietly but causes a lot of harm. You often don't notice it until the damage is already done.
adj. proceeding in a gradual, subtle way, but with very harmful effects. Often used to describe diseases or social problems that develop unnoticed.
The disease is insidious because it has no early symptoms.
High blood pressure is often called an insidious condition because it damages the heart without causing obvious pain.
The most insidious forms of propaganda are those that blend seamlessly into entertainment, slowly altering public perception without ever triggering a critical response.
From Middle French insidieux, from Latin īnsidiōsus (“cunning, artful, deceitful”), from īnsidiae (“a lying in wait, an ambush, artifice, stratagem”) + -ōsus, from īnsideō (“to sit in or on”), from in (“in, on”) + sedeō (“to sit”).
Often used predicatively after linking verbs like 'be' or 'become', or attributively before abstract nouns like 'influence' or 'threat'.